U.S. Pat. No. 7,720,121 to Peng et al. states in the Abstract thereof: “High-power, diode-pumped solid state (DPSS) pulsed lasers are preferred for applications such as micromachining, via drilling of integrated circuits, and ultraviolet (UV) conversion. Nd:YVO 4 (vanadate) lasers are good candidates for high power applications because they feature a high energy absorption coefficient over a wide bandwidth of pumping wavelengths. However, vanadate has poor thermo-mechanical properties, in that the material is stiff and fractures easily when thermally stressed. By optimizing laser parameters and selecting pumping wavelengths and doping a concentration of the gain medium to control the absorption coefficient less than 2 cm-1 such as the pumping wavelength between about 910 nm and about 920 nm, a doped vanadate laser may be enhanced to produce as much as 100 W of output power without fracturing the crystal material, while delivering a 40% reduction in thermal lensing.”
U.S. Pat. No. 7,203,214A to Butterworth discloses a “laser comprising: a laser resonator including a gain element of Nd:YV04 having a length of at least 5 mm, said gain element being end-pumped and wherein the pump-light has a wavelength selected to be different from the peak absorption wavelength of the gain element and falling between about 814 and 825 nanometers in order to reduce thermal stresses and breakage of the gain element, such that the pump source can be operated to deliver greater than 22 Watts of power to the gain medium.”
The publication entitled, Power Scaling of Diode-Pumped Nd:YVO4 Lasers, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 38, NO. 9, SEPTEMBER 2002, Xiaoyuan Peng, Lei Xu, and Anand Asundi, is incorporated herein by reference hereto in its entirety. This article includes information about Nd doping concentrations and it also includes information about the size of the cross-sectional areas of the gain mediums insofar as their ability to handle pump power levels.
Slabs and rods have been made using diffusion bonded techniques for some time. The primary use in Vanadate is to bond undoped end caps to a high gain doped section to reduce thermal lens effects and increase damage threshold. See www.ulticrystal.com and U.S. Pat. No. 5,936,984, issued Aug. 10, 1999 to Meissner et al. Vanadate can only be grown in lengths up to about 30 mm. Most are used as individual components for end pumped gain assemblies Oscillator or amplifier. This essentially requires mode matching between the pump source and propagating beam.
U.S. Pat. No. 5,936,984, issued Aug. 10, 1999 to Meissner et al. states: “A common problem with lasers in general, and with solid state lasers in particular, is the temperature rise in the laserable medium. This effect is the result of only a percentage of the applied pump energy being converted into useful laser radiation while at least a portion of the remaining energy is transferred into competing mechanisms such as radiationless transitions and absorption by the host lattice. To counteract the thermal effects generated by the pump radiation, typically the laserable medium must be cooled.
Cooling the entire laserable medium of a solid state laser presents a difficult problem. First, the cooling fixture should allow as much of the laserable medium to be cooled as possible. Typically some portion of the laserable medium must reside outside of the cooling fixture in order to create a leak-free joint between the fixture and the medium. Second, the cooling fixture should not interfere with the delivery of pump radiation to the laserable medium. Lastly, even if the entire length of the laserable medium is cooled, the optical coatings on the uncooled end surfaces are not cooled and are therefore subject to degradation due to temperature effects, an effect which is aggravated by the high power density of the incident pump radiation.
Some of the effects noted above which result from non-uniform cooling are further exacerbated in end-pumped laser systems. In this type of laser a lens duct is preferably used to homogenize, focus, and concentrate the radiation from the pump source into the laser rod. The end of the laser rod which is coupled to the lens duct is uncooled. Unfortunately, given the high intensity pump radiation passing through the end face of the laser rod and assuming that the laser rod is doped throughout with lasing ions, the ends of the laser rod can experience a substantial rise in temperature during lasing. This rise in temperature can negatively impact the performance of the laser by accelerating the degradation in the optical coatings, increasing the ground state re-absorption losses in quasi-three-level lasers, and eventually leading to the catastrophic failure of the laser due to thermally induced stress fractures.”
Undoped end caps on a short gain medium rod can reduce thermal effects by removing some of the heat from the end faces of the doped part and into the undoped end caps. This reduces the temperature of the gain medium and it also reduces the tendency for thermal fracture of the gain medium.
Picosecond laser design typically utilizes end pumped technology with various amplifier strategies to develop the required gain characteristics. The issue with mode matching is potential heating damage to coatings and bulk rod materials especially as you increase average power and pulse energy. The area of concern is always the last stage where maximum gain and pulse energy occurs. Typical requirements for a relayed beam are spot sizes <0.5 mm in diameter. This leads to potential surface coating and bulk material damage. Obviously, the diameter of the mode matched beam through the gain regions can be increased but this is at the expense of performance.
Typically laser damage consists of point defects in the materials or coatings, inclusions, poor surface preparation or just a lack of cleanliness. Otherwise damage of the coatings or the bulk materials is caused by exceeding the heating damage thresholds of the coatings or bulk materials in the laser.
High power ultrafast technology (picosecond technology) has been developed using a variety of technologies. Typically an ultrafast seed of low power is amplified through a number of stages. A combination of fiber and solid state techniques is employed with methods including cavity dumping and regenerative amplifiers, as examples. Commercially available products are typically limited to pulse energy of 2000 which produces average maximum power of 40 W at a typical repetition rate of 200 kHz.
Typically the exit of the power amplifier of the multistage picosecond is the weak link. End pump amplifiers are mode matched though this region to maximize efficiency. The issue is potential damage to the coatings on surfaces of various optic devices which relay spot dements having diameters in the range of 0.3 mm and 0.6 mm diameter. Further, the issue is also damage to bulk devices.